This invention relates generally to multiple imaging systems and more particularly to a method and system for projecting film images directly onto photosensitive printing plates.
U.S. Pat. No. 3,998,546 which issued on Dec. 21, 1976 to Joseph H. Wally, Jr., et al. discloses a multiple imaging apparatus which has performed well using film as a medium. As described in the patent, the printing industry typically prepares printing plates by means of contact printing techniques utilizing film sheet assemblages known in the industry as film flats. The images that are to be printed are arranged side by side in rows and columns or other configurations on the film flat, and the images may all be the same or they may all be different from one another. In any event, the film flats are prepared by combining the desired images into the large sheets.
Although this type of apparatus has been used extensively and has been generally successful, the time and cost involved in preparing the printing plates are increased due to the two step process that is involved, namely, first projecting the images onto the film flats and then using the film flats to transfer the images by contact onto the printing plates. We have found that by using the present invention and its novel method and combination of components, the intermediate step of preparing film flats can be eliminated, along with the associated delay and cost. To our knowledge, there has not been a practical system available for imposing and assembling images directly onto a printing plate in a manner to successfully produce finished pages which include four color 150 line screen (or finer) halftone color separations with a level of quality as high as present contact methods.
The present invention provides such a system for the first time and has economic advantages in many applications over existing multiple imaging systems such as that shown in the aforementioned Wally, et al. patent. In accordance with the invention, a heavy duty support structure includes an overhead track from which the various components of the imaging system are suspended. High intensity, narrow-banded light is emitted by a high powered mercury arc lamp whose efficiency is heightened by a parabolic reflector. The high intensity light falls onto a dichroic mirror which reflects the light within a relatively narrow wave length band, such as 400-440 nanometers. The narrow banded light beam is collimated by a lenticular (fly's eye) optical integrator. The components of the high intensity light source are contained in a lamp house which is suspended from the overhead track and which can be adjusted along the length of the track by means of a motor operated screwdrive system.
The copy (usualy a film negative or positive) that is to be projected is loaded into an open faced and transparent subject holder which is likewise suspended from the track and adjustable along its length. The subject holder is affixed to a large condenser lens system which is positioned adjacent to the copy between it and the light source. The large condenser lenses intercept the diverging beam of collimated light and converge it through the copy on the subject holder toward an objective lens. After passing through the copy, the light is focused onto the objective lens and the image is projected by it onto the printing plate which is held by a vacuum platen at one end of the support structure. The printing plate is coated with a photosensitive emulsion having an exposure time that is short enough to make the process commercially practical (i.e., capable of being correctly exposed in a very few seconds, not in minutes).
We have found that the high resolution necessary to produce high quality printing can be obtained by using a high intensity, narrow-banded light source in combination with a large condenser lens system located adjacent to the subject holder.